diff --git a/docs/releases.md b/docs/releases.md
index e4f78db..022d5f9 100644
--- a/docs/releases.md
+++ b/docs/releases.md
@@ -25,6 +25,7 @@ and the [ntfy Android app](https://github.com/binwiederhier/ntfy-android/release
 **Documentation**:
 
 * [Examples](examples.md) for [Home Assistant](https://www.home-assistant.io/) ([#282](https://github.com/binwiederhier/ntfy/pull/282), thanks to [@poblabs](https://github.com/poblabs))
+* Install instructions for [NixOS/Nix](https://ntfy.sh/docs/install/#nixos-nix) ([#282](https://github.com/binwiederhier/ntfy/pull/282), thanks to [@arjan-s](https://github.com/arjan-s))
 
 -->
 
diff --git a/server/server_firebase_test.go b/server/server_firebase_test.go
index 8e08b0d..a458f2e 100644
--- a/server/server_firebase_test.go
+++ b/server/server_firebase_test.go
@@ -8,6 +8,7 @@ import (
 	"github.com/stretchr/testify/require"
 	"heckel.io/ntfy/auth"
 	"strings"
+	"sync"
 	"testing"
 )
 
@@ -29,6 +30,7 @@ func (t testAuther) Authorize(_ *auth.User, _ string, _ auth.Permission) error {
 type testFirebaseSender struct {
 	allowed  int
 	messages []*messaging.Message
+	mu       sync.Mutex
 }
 
 func newTestFirebaseSender(allowed int) *testFirebaseSender {
@@ -37,7 +39,10 @@ func newTestFirebaseSender(allowed int) *testFirebaseSender {
 		messages: make([]*messaging.Message, 0),
 	}
 }
+
 func (s *testFirebaseSender) Send(m *messaging.Message) error {
+	s.mu.Lock()
+	defer s.mu.Unlock()
 	if len(s.messages)+1 > s.allowed {
 		return errFirebaseQuotaExceeded
 	}
@@ -45,6 +50,16 @@ func (s *testFirebaseSender) Send(m *messaging.Message) error {
 	return nil
 }
 
+func (s *testFirebaseSender) Messages() []*messaging.Message {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	messages := make([]*messaging.Message, 0)
+	for _, m := range s.messages {
+		messages = append(messages, m)
+	}
+	return messages
+}
+
 func TestToFirebaseMessage_Keepalive(t *testing.T) {
 	m := newKeepaliveMessage("mytopic")
 	fbm, err := toFirebaseMessage(m, nil)
@@ -311,15 +326,15 @@ func TestToFirebaseSender_Abuse(t *testing.T) {
 	visitor := newVisitor(newTestConfig(t), newMemTestCache(t), "1.2.3.4")
 
 	require.Nil(t, client.Send(visitor, &message{Topic: "mytopic"}))
-	require.Equal(t, 1, len(sender.messages))
+	require.Equal(t, 1, len(sender.Messages()))
 
 	require.Nil(t, client.Send(visitor, &message{Topic: "mytopic"}))
-	require.Equal(t, 2, len(sender.messages))
+	require.Equal(t, 2, len(sender.Messages()))
 
 	require.Equal(t, errFirebaseQuotaExceeded, client.Send(visitor, &message{Topic: "mytopic"}))
-	require.Equal(t, 2, len(sender.messages))
+	require.Equal(t, 2, len(sender.Messages()))
 
 	sender.messages = make([]*messaging.Message, 0) // Reset to test that time limit is working
 	require.Equal(t, errFirebaseQuotaExceeded, client.Send(visitor, &message{Topic: "mytopic"}))
-	require.Equal(t, 0, len(sender.messages))
+	require.Equal(t, 0, len(sender.Messages()))
 }
diff --git a/server/server_test.go b/server/server_test.go
index d05075f..ce63f27 100644
--- a/server/server_test.go
+++ b/server/server_test.go
@@ -63,10 +63,10 @@ func TestServer_PublishWithFirebase(t *testing.T) {
 	msg1 := toMessage(t, response.Body.String())
 	require.NotEmpty(t, msg1.ID)
 	require.Equal(t, "my first message", msg1.Message)
-	require.Equal(t, 1, len(sender.messages))
-	require.Equal(t, "my first message", sender.messages[0].Data["message"])
-	require.Equal(t, "my first message", sender.messages[0].APNS.Payload.Aps.Alert.Body)
-	require.Equal(t, "my first message", sender.messages[0].APNS.Payload.CustomData["message"])
+	require.Equal(t, 1, len(sender.Messages()))
+	require.Equal(t, "my first message", sender.Messages()[0].Data["message"])
+	require.Equal(t, "my first message", sender.Messages()[0].APNS.Payload.Aps.Alert.Body)
+	require.Equal(t, "my first message", sender.Messages()[0].APNS.Payload.CustomData["message"])
 }
 
 func TestServer_SubscribeOpenAndKeepalive(t *testing.T) {